1. Field of the Invention
Embodiments of the invention generally relate to electronics, and in particular, to communications circuits.
2. Description of Related Art
Receivers for wire-line applications often must accommodate a wide range of common-mode operating voltages when interfacing the receiver of one integrated circuit (IC) to the transmitter of a possibly different IC. Often, AC-coupling or capacitive coupling implemented with capacitors is used to accommodate differences in the input and output common-mode levels of the receiver and transmitter of the link. These AC-coupling capacitors take board space. In addition, AC-coupling capacitors can restrict the maximum run length of consecutive identical symbols in a transmitted pattern because they have a minimum cutoff frequency below which transmission is blocked. Furthermore, the AC-coupling capacitors can store DC by accumulating charge. Thus, a DC-imbalance in the transmitted signal can be stored on the capacitors and can interfere with the operation of the receiver. A receiver front-end circuit that may be DC-coupled and which is compatible with a wide-input common-mode range is therefore desirable.
For applications involving the transmission of multi-gigabit per second rate NRZ data, there are a multitude of standards which encompass a wide range of required input and output common-mode levels. Some of these standards specify DC-coupled receiver operation and do not allow the use of AC-coupling capacitors.
Finally, in many applications using AC-coupling, because of limitations posed by device biasing requirements of standard receiver analog front end (AFE) circuits, the required receiver input common-mode level is lower than the IC supply voltage. To accommodate this requirement, an additional ‘input termination’ supply is often explicitly called for or is generated internally.